test(desktop): Playwright electron e2e harness

Launches the built app (out/main/index.js) via Playwright's _electron
API — no browser project, no chromium download. Adds a launch fixture
(electronApp/window/app), an AppPage page object with a screenshot
helper for autonomous visual review, and a boot smoke suite (shell
renders, @commitea/core label-parse runs in the renderer, preload API
exposed). Scripts: e2e (build+run), e2e:only, e2e:report. Artifacts
gitignored.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -25,6 +25,20 @@ yarn test       # unit tests (vitest)
 yarn typecheck
 ```
 
+### End-to-end (Electron + Playwright)
+
+```sh
+yarn workspace @commitea/desktop e2e        # build, then drive the built app
+yarn workspace @commitea/desktop e2e:only   # reuse existing out/ build (tight loop)
+yarn workspace @commitea/desktop e2e:report # open the last HTML report
+```
+
+Tests launch the built app (`out/main/index.js`) through Playwright's
+`_electron` API — no browser project, no chromium download. Fixtures and page
+objects live in `apps/desktop/e2e/`; screenshots land in
+`e2e/.artifacts/screens/` for visual review. Page objects use user-facing
+locators (`getByRole`/`getByText`), never CSS/DOM structure.
+
 ## Conventions
 
 - Yarn 4 workspaces; ESM everywhere; `.js` extensions on relative imports